1. Field of the Invention
The present invention relates to an oxygen concentration sensor for an internal combustion engine.
2. Description of Background Information
Systems for controlling the air/fuel ratio by feedback operation have been developed, in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor and the air/fuel ratio of mixture to be supplied to the engine is feedback controlled to a target value in response to an output signal level of the O.sub.2 sensor for the purpose of purifying the exhaust gas and improving the fuel economy.
In such an air/fuel ratio control system, there is a type of oxygen concentration sensor which is capable of producing an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine. As an example, Japanese Patent Application laid open No. 59-192955 discloses an oxygen concentration sensor which includes a pair of flat oxygen ion conductive solid electrolyte members each of which is provided with a pair of electrodes, and arranged so that the measuring gas is retained in the proximity of the surface of one of electrodes provided on one of said pair of solid electrolyte members, and the surface of the other electrode provided on that one of two solid electrolyte members faces an atmospheric air.
In this oxygen concentration sensor, one of the two oxygen ion conductive solid electrolyte members and the electrode pair operate as an oxygen concentration ratio detection sensor cell element, and the other one of two oxygen ion conductive solid electrolyte members and the electrode pair operate as the oxygen pump element. By supplying a current so to cause the oxygen ions to move through the inside of the oxygen pump element toward the electrode located on the side of the retained measuring gas when the voltage generated across the electrodes of the oxygen concentration ratio detection sensor cell element is higher than a reference voltage, and to cause the oxygen ions to move through the inside of the oxygen pump element toward the electrode located on the other side of the retained measuring gas when the voltage generated across the oxygen concentration ratio detection sensor cell element is equal to or lower than the reference voltage, the pump current value becomes proportional to the oxygen concentration (air/fuel ratio) both in the lean and rich regions. FIG. 1 shows an example of the pump current I.sub.P which varies in proportion to the oxygen concentration. This pump current I.sub.P is in general detected in the form of a voltage generated across the terminals of a current detection resistor which is provided in series with the oxygen pump element.
Furthermore, in this type of oxygen concentration sensor, a desirable characteristic of proportional oxygen concentration detection is obtained only when the temperature of the oxygen pump element and the sensor cell element are raised quite high. Therefore, a heater element fixed on the oxygen ion conductive solid electrolyte is generally used, so that the oxygen pump element and the sensor cell element are heated by the heater element. In the normal structure, one of the electrodes of the oxygen pump element is grounded and the voltage is applied to the other electrode through the current detection resistor. In this structure, the heater element is supplied with a heater current. However, there is a problem of a part of the heater current supplied to the heater element flowing into the oxygen pump element by leakage this in addition to the pump current normally supplied to the oxygen pump element. This leakage of the pump current influences the amount of moving ions. Therefore, from the voltage across the terminals of the current detection resistor under such a condition, it is difficult to accurately detect the pump current value, i.e. the oxygen concentration in the measuring gas.